1. Field of the Invention
This invention relates generally to inflatable structures, and more particularly to inflatable tubular structures with helical seams.
2. Description of the Related Art
It is known in the prior art to construct inflatable structures of several pieces of material adhesively bonded together to form tubular members impervious to air and water. Such inflatable tubular members are often utilized in water floatation devices such as life rafts and inflatable evacuation devices for commercial aircraft.
As illustrated in FIGS. 1 and 2, a body of a prior art inflatable annular floatation device or raft 10 comprises four tubular segments 12, 14, 16 and 18 that are constructed of water and air impervious material. The tubular elements are adhesively bonded together at transverse circular seams 20, 22, 24, and 26. Each segment is arcuate in shape and includes a lower wall 28 adhesively bonded to an upper wall 30 at an inner seam 32 and an outer seam 34. Thus, the annular floatation device 10 comprises a total of twelve adhesively bonded seams that may be subject to leakage due to improper assembly, over-inflation of the floatation device 10, cyclic loading and unloading due to inflation and deflation, wear, and the like. Overlapping or cross-over seam areas at the intersections of the inner and outer seams and the transverse seams are especially vulnerable to leakage.
During construction of the prior art floatation device 10, and with additional reference to FIG. 3, each segment 12, 14, 16 and 18 is manually formed by cutting woven material such that the warp 36 of the material is oriented parallel to the longitudinal axis 40 of the tubular member and the fill 38 is oriented transverse to the longitudinal axis. As shown, the warp 36 extends parallel to the seams 32 and 34 in order to prevent twisting of the floatation device 10 during inflation and operation. However, such warp direction requires special orientation of fabric during the cutting which further complicates the assembly process and results in a significant amount of material waste. Moreover, each wall 28, 30 of each segment must be laid out separately prior to applying adhesive. A substantial amount of labor is therefore required to piece the walls and segments together.
The above-described problems are further augmented by the long curing time of adhesives used to bind the seams together. The formation of each seam requires the application of a suitable layer of adhesive between overlapping areas of the walls and segments. Each seam must be formed separately and typically must be cured for at least four hours before forming a subsequent seam, which also must be cured for at least four hours, and so on. Inflating and testing the floatation device 10 cannot take place until the final seam is completely cured. With a total of twelve seams for the prior art annular floatation device 10, much manufacturing time is lost due to the long curing process.
Another drawback associated with this prior art inflatable arrangement is illustrated in FIG. 4. As shown, the outer seam 34 and inner seam 32 of each tubular segment 12, 14, 16 and 18 are subject to a circumferential force F.sub.c when the device 10 is inflated. The circumferential force F.sub.c extends normal to the seams 32 and 34 and therefore equals the normal force F.sub.n acting on the seams. The normal force F.sub.n tends to pull the seams 32 and 34 apart. This is especially prevalent when the device 10 is over-inflated or when too much external force is applied to the walls of the inflatable device 10.